Evidence for the transmission of HIV from mother to fetus via the placenta has been documented but the mechanism of transmission is unknown. The overall aim of this proposal is to determine the cellular and molecular mechanisms involved in the placental transport of HIV from maternal blood to the fetus. This objective will be pursued by taking advantage of our ability to obtain pure population of human trophoblast cells and our ability to demonstrate cell-mediated infection of these cells in vitro. Placental syncytiotrophoblast is in direct contact with maternal blood and virus must cross this epithelium before reaching stromal cells, endothelial cells and, eventually, the fetal bloodstream. The first specific aim will focus on characterizing the role of lymphocytic/monocytic cells in the cell-mediated infection of syncytiotrophoblast. We will use immunocytochemical and transmission and scanning electron microscopic techniques to study the kinetics of the cell-cell interaction and the infection process. The cellular pathway by which HIV enters the cells will be defined. The possibility that cytokines or other factors released by lymphocytic/monocytic cells play a role in the cell mediated infection process will be studied. Other experiments will establish the nature of the adhesion molecules involved in cell-cell binding and analyze the role of antibody and CD4 in the infection process. In the second specific we aim will attempt to determine whether the interaction of HIV with syncytiotrophoblast varies as a function of gestational age by characterizing the nature of HIV infection of cells obtained from first and second trimester placentas. In the third specific aim we will determine whether anti-viral dideoxynucleosides prevent HIV infection of placental cells and whether cell function is compromised. In the last specific aim we will focus on placental macrophages, fibroblasts and endothelial cells. As well as characterizing viral internalization and release we will examine in vitro infected cells for alterations in the secretion of cytokines and growth factors. Information gained should be useful in designing strategies to prevent HIV transmission.